1. Field Of The Invention
The invention relates generally to a method for forming thin liquid phase epitaxy (LPE) crystalline layers of silicon having less than 5X10.sup.16 Cu atoms/cc impurity by providing a saturated solution of Si in Cu at about 18%.-40% by weight Si at temperatures ranging between about 800.degree. C. about 1150.degree. C.; immersing a silicon substrate in the saturated solution; lowering the temperature of the saturated solution and holding the immersed substrate in the solution for a period sufficient to cause Si to precipitate out of the solution to form a crystalline layer of Si on the substrate; and withdrawing the substrate from the solution.
Growing ingots, wafering, and thin-polishing is not cost effective in providing thin silicon layers and the wafers are fragile to handle. Ribbons of silicon are difficult to grow in such thin geometries and the ribbons are also fragile. On the other hand, growing thin layers on a substrate from the Si melt requires high temperatures and hence high energy input. Furthermore, due to the surface tension of Si, thin flat layers can only be grown on substrates which the silicon wets well, such as graphite; however, this can introduce impurity problems. Moreover, the substrates must be able to withstand a temperature of 1430.degree. C. and small Si grain sizes generally result.
Epitaxial growth of Si on a substrate from a gas phase by chemical vapor deposition is uneconomical because of slow growth rates and plasma vacuum, and sputtering deposition techniques generally result in too small grain sizes.
Therefore, liquid phase epitaxial growth of Si would appear more suitable, if solvent impurities can be adequately avoided.
The method of the present invention entails utilizing LPE by using an improved solvent of Cu in a manner to produce thin crystalline layers of device quality silicon faster, at lower process temperatures, and with improved control of dopant content.
2. The Prior Art
U.S. Pat. No. 4,201,623 discloses a crystal substrate of &lt;111&gt; silicon doped by exposure to a liquid metal solvent, wherein the substrate is carried in a cavity in a refractory boat, and the solvent is carried in a perforation of a cover for the boat; said boat being heated to a certain temperature in a non-oxidizing atmosphere and moved to place the substrate cavity under the cover perforation so that the solvent and substrate come into contact.
A process for growing thin epitaxial layers of Group III-V semiconductor materials is described in U.S. Pat. No. 4,088,514. The process prepares a saturated solution of the semiconductor materials in a metal melt by keeping an under-saturated solution in contact with the crystalline semiconductor material at a predetermined temperature, supercooling the saturated solution and bringing the supercooled solution into contact with a substrate.
U.S. Pat. No. 2,990,372 describes a process for producing silicon of high purity by dissolving silicon in a metal and crystallizing the silicon therefrom. The silicon is dissolved at a temperature in the range of about 700.degree.-1200.degree. C. in a metallic melt used in an amount sufficient for dissolution of the silicon and suitable for crystallization of the silicon upon cooling, wherein the metal melt is of a melt-forming metal and alloys of these metals; including in the melt a dope for the silicon semiconductor; slowly reducing the temperature of the melt including the silicon and dope in order to effect crystallization of the silicon and dope and solidification of the melt; and separating the crystallized dope-containing silicon by dissolving the non-silicon ingredients of the solidified melt in an acid, the melt-forming metal of the metallic melt-silicon solution being present as the principal component of the solution based on the metal-forming metal and silicon content of the solution.
A method of purifying silicon is disclosed in U.S. Pat. No. 4,822,585 which comprises: providing a molten body of a silicon material in a solvent metal of copper; extracting heat from the body to provide a solid phase containing silicon in crystal form and to concentrate impurities in a molten phase; removing a substantial part of the molten phase from the solid phase containing the silicon crystals; subjecting the solid phase to a melting action to melt at least a fraction of the solid phase for purposes of removing a substantial amount of copper-silicon combination adhering to the crystals; and separating at least a fraction of the melted material from the crystals of silicon.
In prior art processes for forming crystalline silicon, there is the requirement of holding the Si melt at relatively high temperatures and this occasions high-power requirements and the higher costs associated therewith. Moreover, in prior art processes for forming crystalline silicon that involve silicon solvents of Ga or In, these processes have suffered from a lack of control over the levels of particular dopant that is desired to be incorporated in the crystalline silicon product.
Liquid phase epitaxy (LPE) is also sometimes called high-temperature solution growth or metal solution growth (if the solvent is a metal), and it has been used since the 1960's in the semiconductor industry for group III-V growth (GaAs, GaP, etc.). Thin layer silicon epitaxy has also been heavily investigated using the solvents of group III's (Al, Ga, In); group V's (Sb, Bi); group IV (Sn); and the metals Au, Cd, Pb, and Ag.
However, a problem with group III and V solvents is that they are also electrical dopants for silicon, and usually lead to difficulties with dopant control. Many metals degrade the minority charge carrier lifetime of the silicon, which is detrimental for most applications. These problems arise because there is some solubility of the solvent element in the grown silicon layer, and this varies with the particular solvent.
A problem with some solvents is that they do not dissolve much silicon and this can lead to excessively slow growth rates.
In the early 1980's, Olson, Carleton, and Tejedor showed the advantageous property of solid Cu-Si alloys to preferentially retain impurities, and used them as source material for bulk polycrystalline Si formation via chemical vapor deposition and electrodeposition of silicon. They were able to refine silicon by both electrodeposition (J. M. Olson and K. L. Carleton, J. ElectroChem. So. 128 No. 12 (1981) 2699) and by chemical vapor deposition (P. Tejedor and J. M. Olson, J. of Crystal Growth 89 (1988) 220). They were also able to achieve bulk silicon with typically less than 5 ppm impurity compared to their silicon starting material which contained about 1% impurity.
In U.S. Pat. No. 4,822,585,Cu solvent was used to advantage in a liquid phase purification process for bulk silicon.
Therefore, a need exists to provide a method for producing thin crystalline layers of device quality silicon faster, at lower process temperatures, and with improved control of the dopant content.